1. Field of the Invention
The present invention relates to deflection yokes and color cathode ray tubes with the deflection yokes.
2. Description of the Related Art
In the current color cathode ray tubes used in a display monitor as windows, information is very often displayed in the peripheral area of the screen. Therefore a technology enabling minute image display in such area is being called for. Since the raster distortion is an essential element in determining the image quality in the peripheral area of the screen, the standards for the pincushion distortion in the upper and lower edges of the screen or for the raster distortion which depend on the magnetic field distribution of the deflection yoke itself have become very demanding. Further, the demand to the convergence in the peripheral area of the screen has become very severe as well.
A self-convergence type deflection yoke used in a cathode ray tube having an inline electron gun comprises a saddle shaped horizontal deflection coil 24, a saddle shaped vertical deflection coil 25 located outside the horizontal deflection coil 24, and a high permeability core 26 located outside the vertical deflection coil 25 as illustrated in FIGS. 22 and 23. In such a self-convergence type deflection yoke, the magnetic field of the horizontal deflection coil 24 is designed to form a pincushion shape and the magnetic field of the vertical deflection coil 25 is designed to form a barrel shape in order to correct both the pincushion distortion at the upper and lower edges of the screen and the misconvergence on the screen at the same time.
With the trend of enlarging the curvature of cathode ray tubes in recent years, a "positive anisotropic astigmatism" as illustrated in FIG. 24 tends to emerge on the screen and at the same time the pincushion distortion at the upper and lower edges of the screen tends to increase.
The "positive anisotropic astigmatism" will be explained. In FIG. 24, the letters B, G, R, denote three electron beam irradiation sources viewed from the screen side. The broken line 27 denotes the blue pattern of the electron beam irradiated from the electron beam irradiation source B, the chain line 28 the red pattern of the electron beam irradiated from the electron beam irradiation source R, and the solid line 29 the green pattern of the electron beam irradiated from the electron beam irradiation source G, respectively. In the first quadrant of the upper right of the screen, the red pattern (chain line) 28 emerges to the downward and the blue pattern (broken line) 27 to the upward with respect to the green pattern (solid line) 29, with the red pattern (chain line) 28 and the blue pattern (broken line) 27 crossing on the vertical axis to form an X shape. In the second quadrant of the upper left of the screen, the position of the red pattern (chain line) 28 and the blue pattern (broken line) 27 is reversed with respect to the first quadrant. In the lower half of the screen, the position of the patterns is symmetrical with the horizontal axis as the line of symmetry. This is called the "positive anisotropic astigmatism".
In conventional self-convergence type deflection yokes, if the magnetic field of the vertical deflection coil 25 is formed as a stronger barrel shaped magnetic field to correct the positive anisotropic astigmatism in the screen, the pincushion distortion at the upper and lower edges of the screen further increases. Besides, if the magnetic field of the horizontal deflection coil 24 is formed as a stronger pincushion shaped magnetic field to correct the pincushion distortion at the upper and lower edges of the screen, the positive anisotropic astigmatism tends to further increase. Therefore, it is impossible to correct both the pincushion distortion at the upper and lower edges of the screen and the misconvergence of the screen at the same time.
In a deflection coil used in a deflection yoke, the magnetic field distribution from the screen side toward the electron gun side is concerned with the misconvergence correction on the screen as a whole, while the magnetic field distribution of the deflection coil at the screen side is concerned with the pincushion distortion at the upper and lower edges of the screen. This is because the distance between the electron beam and the deflection coil at the screen side is shorter than that at the electron gun side when deflecting the electron beam, and the effect of the magnetic field distribution of the screen side on the pincushion distortion at the upper and lower edges of the screen becomes greater at the screen side of the deflection coil for the electron beam passing on the tip of the curve of lines of magnetic force.
As heretofore mentioned, in order to correct the pincushion distortion at the upper and lower edges of the screen by means of a deflection yoke, the pincushion magnetic field at the screen side of the deflection coil should be strengthened. Further, in order to correct the misconvergence on the screen in the condition, the barrel magnetic field at the vicinity of the middle part and the electron gun side excluding the screen side of the deflection coil should be strengthened.
In order to meet such requirements, a method of achieving both the correction of the pincushion distortion at the upper and lower edges of the screen and the convergence by further providing correction magnets at the upper and lower parts of the screen side of the deflection yoke has been advocated as disclosed in the Japanese Patent Application Laid Open No. 204947/1990.
In a self-convergence type deflection yoke, the magnetic field of the horizontal deflection coil 24 has a strong pincushion distortion in order to eliminate the raster distortion at the upper and lower edges of the screen by designing the magnetic field distribution of the deflection yoke itself (see FIG. 14). However, when much fifth-order pincushion distortion is included therein, a high order raster distortion at the upper and lower edges called gullwing is generated. Since the gullwing deteriorates the visual image quality drastically, it should be prevented.
In order to meet such demands, a method of reducing gullwing at the upper and lower edges of the screen by forming a dent at the center of the screen side flange of the horizontal deflection coil is proposed in the U.S. Pat. No. 4,233,582. Another method of reducing the gullwing at the upper and lower edges of the screen by having the screen side flange of the horizontal deflection coil in a polygonal shape is advocated in the U.S. Pat. No. 4,229,720. Further, a method of reducing the gullwing at the upper and lower edges of the screen by providing correction magnets with a protruding part at the upper and lower parts of the screen side is proposed in the Japanese Patent Application Laid Open No. 289748/1988.
However, in the method disclosed in the Japanese Patent Application Laid Open No. 204947/1990, since the method aims at both the correction of the pincushion distortion at the upper and lower edges of the screen and the convergence by providing correction magnets, there are problems such as an increased number of parts, and the wide variation of the magnetization of correction magnets in the production process.
In the method disclosed in the U.S. Pat. No. 4,233,582, in the pressing process to provide a dent at the center of the screen side flange of the horizontal deflection coil, the excessive stretching of the coil wire could damage its insulation coating layer. Further, if a dent is formed too deep, since the dent comes in contact with the funnel portion of the cathode ray tube when the deflection yoke is attached to a cathode ray tube, there is a problem in production or designing that it is difficult to form a dent sufficient to eliminate the gullwing. Further, in the method disclosed in the U.S. Pat. No. 4,229,720, there is a problem in production in that a coil wire is liable to be deformed and damaged at the apexes of the polygon-shaped screen side flange of the horizontal deflection coil. In the method disclosed in the Japanese Patent Application Laid Open No. 289748/1988, there are problems such as the increased number of parts by providing correction magnets, or the wide variation of magnetization of correction magnets in the production process.